1. Field of the Invention
The present invention relates to silver (Ag) alloy reflective films for optical information recording media, Ag alloy sputtering targets for the deposition thereof, and optical information recording media. More specifically, it relates to reflective films which have low thermal conductivities, low melting temperatures, high reflectivities, and high corrosion resistance and therefore enable marking on optical information recording media such as CDs, DVDs, Blu-ray Discs, and HD-DVDs typically using laser, such as marking of identification numbers in accordance typically to the burst cutting area (BCA) specifications, after the preparation of the media. The present invention also relates to sputtering targets for the deposition of the reflective films, and optical information recording media having the reflective films.
2. Description of the Related Art
Optical information recording media (optical discs) include various types, and the three main types categorized by the writing/reading system are read-only, write-once, and rewritable optical discs.
Of these optical discs, read-only discs have recorded data formed by concave and convex pits on a transparent plastic substrate such as a polycarbonate substrate upon the manufacture of the discs, and a reflective layer mainly containing, for example, Al, Ag, or Au is arranged on the recorded data, as illustrated in FIG. 1. The data are read out by detecting phase difference or reflection difference of laser beam applied to the discs. Certain optical discs include a substrate containing recording pits and a reflective layer arranged on the recording pits, and another substrate bearing recording pits and a semi-reflective layer arranged on the recording pits. The two substrates are laminated, and the data recorded on the two layers are read out. Data recorded on one side according to this recording/reading system are read-only data that cannot be additionally wrote and altered, and optical discs using this system include CD-ROMs, DVD-ROMs, and DVD-Videos. FIG. 1 is a schematic view of the sectional structure of a read-only optical disc. The optical disc in FIG. 1 includes polycarbonate substrates 1 and 5, a semi-reflective layer (Au, Ag alloy, and Si) 2, an adhesive layer 3, and a total-reflective layer (Ag alloy) 4.
These read-only optical discs are produced in quantities, and information is recorded upon production of the discs by pressing with stampers having patterns of the information. Thus, IDs cannot be significantly given to individual discs. However, read-only optical discs individually having IDs formed using a dedicated system such as a label gate system or a burst cutting area (BCA) system after the preparation of the discs are being standardized typically for preventing unauthorized copying, improving traceability in distribution of products, and increasing added values of products. The ID marking (recording) is mainly carried out by a method of applying laser beam to discs after production to melt an Al alloy in the reflective film and to form holes therein.
Aluminum alloys, such as Al—Mg alloys according to Japanese Industrial Standards (JIS) 6061, are distributed in quantities as general construction materials, are inexpensive and are thereby widely used as reflective films of read-only optical discs. In contrast, Ag alloys having higher reflectivities are widely used in recordable (write-once and rewritable) optical discs.
These materials, however, have high thermal conductivities and require high laser power for marking, which results in damages on base materials including polycarbonate substrates and adhesive layers. Additionally, they have low corrosion resistance, and voids formed as a result of laser marking invite corrosion of the reflective film when held under conditions of high temperatures and high humidity after laser marking. Reflective films containing Ag alloys show decreased reflectivities at high temperatures due to cohesion of Ag reflective films caused by the low thermal stability of Ag. In addition, these Ag alloy reflective films show decreased reflectivities and coloring when they are used typically in read-only media and are exposed to sunlight or fluorescent light over a long time. This is probably because Ag is dissolved or forms a reaction product layer as a result of a reaction at the interface between the reflective films and other components such as polycarbonate substrates.
Japanese Laid-open (Unexamined) Patent Application Publication (JP-A) No. 1992-252440 (Hei 04-252440) discloses a method for reducing the thermal conductivity of an Ag alloy by incorporating Ge, Si, Sn, Pb, Ga, In, Tl, Sb, or Bi into Ag. JP-A No. 1992-28032 (Hei 04-28032) discloses a method for reducing the thermal conductivity of an Ag alloy by incorporating Cr, Ti, Si, Ta, Nb, Pt, Ir, Fe, Re, Sb, Zr, Sn, or Ni into Ag. The resulting reflective films obtained according to these techniques, however, are not intended to be melted and removed by laser irradiation, and some of them show increasing melting temperatures with decreasing thermal conductivities thereof. Silver alloys (Ag alloys) satisfying requirements as Ag alloys for laser marking have not yet been provided.
The present inventors have developed Ag alloys which have improved thermal conductivities, laser-beam absorptivities, and corrosion resistance and are suitable for laser making, by incorporating, for example, Nd, Sn, Gd, and/or In into Ag (Japanese Patent Applications No. 2004-208686 and No. 2005-67262). Thin films using these materials, however, show decreased light stabilities with increasing contents of added elements, although they show reduced thermal conductivities and increased laser-beam absorptivities. Accordingly, demands still have been made to provide materials having both excellent laser marking capabilities and higher resistance to weathering and light.
The laser-beam absorptivity, light resistance, and weather resistance are properties of reflective films acting as follows. Specifically, an increased laser-beam absorptivity enables easier marking of the reflective films by laser beam, i.e., it improves laser marking capability. An increased light resistance inhibits the reduction in reflectivity and the coloring as a result of exposure to sunlight or fluorescent light. An increased weather resistance prevents the reduction in reflectivity due to corrosion or cohesion of Ag under conditions of high temperatures and high humidity. Therefore, Ag alloy reflective films provided for laser marking should have low thermal conductivities, high laser-beam absorptivities, excellent weather resistance, and excellent light resistance.
As is described above, Ag alloy thin films provided for laser marking must have low thermal conductivities, high laser-beam absorptivities, excellent weather resistance, and excellent light resistance.
Current reflective films for read-only optical discs use JIS 6061 series Al alloys, but these Al alloys do not satisfy the requirements for laser marking in thermal conductivity and corrosion resistance (weather resistance).